I think Duncan is spot on.
I have two watches specified to 200m - a Tag Heur and a cheap Vostok Amphibian. I have read reports of people successfully using the Amphibian for scuba diving so I'm reasonably confident they will both cope with swimming, falling out the canoe, etc. Seals must age and degrade so I'd guess the 20 atmosphere pressure test only applies when brand new.
100m watches are usually cheaper than the 200m types. I would expect they would be ok for swimming, but user reports on this would be interesting.
It is indeed curious that military watches are not more highly specified for waterproofness. I wonder, for example, if the G10 survives swimming. I have a Red Army issue Vostok Komandirskie that does not have a specified water resistance but does have a screw in crown. It's fine in the shower but I'm not sure about swimming in it.
I think it's about time the Fair trading people made watch manufacturers stop using '30 metres' or '3 atmospheres' descriptions, and substituted a more realistic 'do not immerse' tag.